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Mystery meteorite found fossilised in quarry rocks

This space rock, unlike any other, gives planetary scientists a glimpse at the early solar system. Belinda Smith reports.

The Oesterplana 065 fossil meteorite (dark grey) lodged in red limestone. The pale grey halo is where oxygen in the limestone was consumed while the meteorite weathered on the sea floor.

Birger Schmitz

A Swedish limestone quarry turned up a rocky revelation – a new type of meteorite, which had been locked away in rock for 470 million years.

Birger Schmitz from Lund University in Sweden and colleagues described the unique piece of space rock, which may help astronomers reconstruct the history of the solar system, in Nature Communications.

Phil Bland, astronomer and meteorite hunter at Curtin University in Western Australia, but wasn't involved with the study, calls it "a lovely piece of work.

"We've found tens of thousands of meteorites on Earth. To find a completely new type – well, it's fantastic."

Some 85% of meteorites are "chondrites" – chunks of rock that formed when dust in the early solar system coagulated into asteroids, which over time smashed into each other and rained onto Earth.

The Thorsberg quarry in southern Sweden is a rich source of chondrite meteorites, with more than 100 dug up in recent decades.

Its 470-million-year-old limestone used as pavers, among other things, but at around the time the rock was forming on the sea floor, the planet was peppered with chondrite meteorites – fragments of, some planetary scientists think, a large asteroid that collided with another object.

But that second object remained enigmatic. No sign of it was found.

Until 2014, when Schmitz reported a strange-looking meteorite, dubbed Oesterplana 065, in the bed of rock the chondrites were found.

Around eight centimetres long and 6.5 wide, the dark grey smudge was discovered locked in limestone layers.

While it didn't look like a typical stony meteorite, thanks to years of weathering, the limestone preserved tiny specks called chromite grains from which its composition information could be extracted.

In this most recent paper, they analysed the different types (or isotopes) of oxygen and chromium in the meteorite and found it unlike anything other.

They also dated the meteorite and showed its age to be within a million years of the age of chondrites from the quarry.

They believe, then, the asteroid that spawned the Oesterplana 065 fragment collided with the chondrites' parent asteroid, but it shattered into tiny pieces in the process, thus explaining why the meteorite is – so far – one of a kind.

Schmitz and colleagues write that Oesterplana 065 "may be the first documented example of an 'extinct' meteorite" – in other words, a type of meteorite that doesn't fall on Earth today because its parent asteroid is now completely destroyed.

Asteroid families – that is, bundles of small asteroids formed when a larger one breaks apart – have a half-life between a billion and 500 million years, Bland says: "This means the asteroid belt" – where most asteroids reside, orbiting the sun between Mars and Jupiter – "constantly cycles through different material."

Finding more extinct meteorites and tracing which types fell on Earth – and when – will help astronomers to piece together clues about our solar system's youth, the researchers write.

"There is potential to reconstruct important aspects of the solar system history by looking down in Earth's sediments, in addition to looking up at the skies."